An eclipse season is a predictable astronomical event, defined as a period when the geometry of the Sun, Earth, and Moon aligns in a way that makes eclipses possible. This alignment occurs because the Sun is positioned near the Moon’s orbital nodes, which are the only places where the three celestial bodies can line up straight enough for a shadow to be cast. The phenomenon is a direct consequence of the Moon’s tilted orbit, which otherwise prevents eclipses from happening every month.
Defining the Eclipse Season
The eclipse season is a temporary window of time, lasting between 34 and 37 days, when the orbital mechanics allow for eclipses to take place. These seasons occur roughly every six months, meaning there are always two, and sometimes three, eclipse seasons in a calendar year. The interval between successive seasons is approximately 173.3 days, which is the time it takes the Sun’s apparent path to move from one orbital node to the next.
Because the time between seasons is less than half a tropical year, the two annual windows of opportunity shift backward slightly on the calendar each year. The timing of the eclipse seasons is not fixed to the solstices or equinoxes, but constantly precesses. This precession causes the entire cycle of eclipse seasons to slowly move through the calendar, taking about 18.6 years to complete a full circuit.
The Role of Orbital Nodes
The existence of the eclipse season depends entirely on the concept of the Moon’s orbital nodes. The Moon’s orbit around the Earth is tilted by about 5.1 degrees relative to the Ecliptic plane (the plane of the Earth’s orbit around the Sun). This inclination means that for most of the month, the New Moon passes above or below the Sun, and the Full Moon passes above or below the Earth’s shadow.
The orbital nodes are the two points where the Moon’s tilted orbit intersects with the Ecliptic plane. For an eclipse to occur, the Sun, Earth, and Moon must align in a straight line, which is known as syzygy. This precise alignment can only happen when the Moon is at or very near one of these two nodes when it is New or Full.
When the Sun’s apparent position is close to one of these nodes, an eclipse season begins. The season ends when the Sun moves too far away from the node for the alignment to be close enough. The 5.1-degree tilt makes the nodes the gating factor for eclipses, ensuring they are rare events instead of occurring every New Moon and Full Moon.
The Types of Eclipses That Occur
Every eclipse season is guaranteed to feature at least two eclipses—one solar and one lunar—which happen about two weeks apart, corresponding to the New Moon and Full Moon phases. Sometimes, if an eclipse occurs very early in the 34-to-37-day season, a third eclipse can happen near the end of the window. These events are divided into two main categories based on which body is casting the shadow.
Solar Eclipses
A solar eclipse happens during the New Moon phase when the Moon passes directly between the Sun and Earth, casting a shadow on our planet. The Moon’s shadow has two parts: the darker inner shadow called the umbra, and the lighter outer shadow called the penumbra. A total solar eclipse occurs when an observer is in the umbra, while a partial solar eclipse occurs when they are in the penumbra, where the Moon only covers a part of the Sun’s disk.
If the Moon is near the farthest point in its elliptical orbit from Earth, its apparent size is slightly smaller than the Sun’s, resulting in an annular solar eclipse, where a bright ring of sunlight remains visible.
Lunar Eclipses
A lunar eclipse, conversely, happens during the Full Moon phase when the Earth passes between the Sun and the Moon, casting its shadow onto the Moon. A total lunar eclipse occurs when the Moon fully enters the Earth’s darkest shadow, the umbra, often giving the Moon a reddish hue due to light refraction through Earth’s atmosphere.
Partial lunar eclipses occur when only a portion of the Moon passes through the umbra. If the Moon passes only through the fainter outer shadow, the penumbra, it creates a penumbral lunar eclipse, which is often difficult to distinguish from a normal Full Moon. The precise type of eclipse—Total, Annular, Partial, or Penumbral—depends directly on how close the alignment of the Sun, Earth, and Moon is to the exact center of the orbital node.